1. Field of the Invention
The present invention generally relates to photo-sensor arrays and more particularly to a unique three-dimensional pixel structure that significantly improves pixel packing density.
2. Description of the Related Art
Semiconductor photosensors have been found in a wide variety of applications. These include position measurement, CMOS imagine sensors, motion detector, image capturing and velocity measurement. One key application of these devices however is for optical-fiber communication.
The basic photo sensing mechanisms, as summarized by S. M. Sze in the text book of Physics of Semiconductor Devices, p. 743 (incorporated herein by reference), are: (1) carrier generation by incident light, (2) carrier transport and/or multiplication by some sort of current-gain devices, and (3) interaction of current and IC circuits to provide output signals. A well-designed photo-sensor provides high sensitivity at operating wavelengths, high response speed, and minimum noise. It is desirable that photo-sensor chips be small in size, reliable under operating conditions, and operated at low power.
From a device aspect, photo-sensors can be presented in many different types, such as p-i-n diode, p-n diode, metal semiconductor diode, metal-i-n diode, etc. In general, p-n diodes have a lower response speed than p-i-n diodes (described in greater detail below). This is because the generated photocurrent consists of large portions of diffusion current and small portions of drift current due to thin depletion region. At long wavelengths, the required absorption depth becomes very long which causes performance of p-n diodes to degrade further.
One of the reasons for the increased performance of p-i-n diodes is that they include a depletion region (or the intrinsic layer) which has a thickness that allows p-i-n diodes to be tailored to optimize quantum efficiency and frequency response. The basic photosensing mechanism of a p-i-n diode has light absorption in the depletion (or i-layer) region that produces hole-electron pairs which will be separated by an applied electric field. The diode is reverse biased, so that electron “holes” drift to the p terminal, which is tied to ground, while electrons drift to the n terminal, which is tied to a positive voltage. This results in higher current flow in the external circuit than that of the p-n diode sensors due to large drift space.
If metal is used to form photosensors, usually it has to be very thin (10 to 20 nm) so that it is semi-transparent to the incident light. In general, metal is also highly reflective and an anti-reflective coating (e.g., 50 nm of ZnS) is necessary to enhance quantum efficiency.
Another application for photosensors is use as an image sensor. Complementary metal oxide semiconductor (CMOS) image sensors have advantages such as low-cost, low-power, and a high level of integration. CMOS image sensor can be used in digital cameras or devices such as motion detectors. In general, each pixel of CMOS image sensor comprises ⅕ circuit area, and ⅘ diode area. Further, in order to ensure sufficient total photon flux, conventional two-dimensional p-n photosensors are inherently designed with large spacing. Therefore, conventional CMOS image sensors have relatively poor pixel density and there is a need to increase the pixel density.